1. Field of Invention
This invention relates specifically to vehicle tire pressure sensing and in general to remote pressure sensing.
2. Description of Prior Art
The U.S. Government has passed a law, known as the TREAD Act, requiring in-dash tire pressure reporting or warning systems for all new vehicles. U.S. Pat. No. 6,662,642 to Breed, et al, provides a good summary of the present art. Two main types of systems have emerged to meet this requirement—indirect and direct pressure sensing systems. The first, or indirect, type measures differential tire rotation speed to detect an anomalous rate for one tire, indirectly indicating under- or over-inflation. The advantage is passive operation with no in-tire components but the disadvantage is the inability to detect anomalous pressure in all tires. The second, or direct, type typically involves placing battery-operated transmitters within tires (possibly attached to or part of the tire valve stem) to transmit pressure readings to external receivers. While this permits sensing the pressure in all tires, the in-tire unit is relatively large due to the requirement for a battery. Another disadvantage is the periodic need to dismount tires to replace batteries. Alternative systems either try to generate sufficient electrical power internally through various means or transmit sufficient power into tires from external sources.
U.S. Pat. No. 6,520,006 to Burns discloses another direct approach. Here, a remote vehicular tire pressure reporting system comprises (1) an in-tire magnetic pressure sending apparatus wherein a permanent magnet is rotated mechanically in response to tire pressure plus (2) a magnetic pressure reading apparatus mounted on the vehicle containing sensors responsive to magnetic field direction. Advantages include passive operation that eliminates the need for battery replacement and sensing the direction rather than strength of a distant magnetic field. Magnetic field direction is more accurately controlled and measured than magnetic field strength. However, this system has the disadvantage of requiring coaxial alignment between the sender and receiver, at least once per wheel rotation, which limits location possibilities for both the sender and the reader. In the present improved invention, a method for reading the orientation of the sender field pressure reader that exploits wheel rotation during all or part of a rotation cycle lifts the restriction of coaxial alignment.
U.S. Pat. No. 6,647,771 also to Burns discloses another magnetically coupled tire pressure reporting system based on a novel magnetooptic display attached to the outer tire wall. However, this system does not meet the requirements for an in-dash pressure display.
U.S. Pat. No. 4,866,982 to Gault teaches a tire pressure monitoring system where a stationary Hall-effect sensor measures tangential spacing between a fixed magnet and a second magnet moveable in response to a linear pressure actuator. Changes in tangential spacing between the two magnets affect the timing between features in the combined magnetic field patter. Variations in timing are determined from the signal waveform generated as the spaced magnets, rotating with a wheel, sweep by a stationary sensor. U.S. Pat. No. 4,807,468 to Galan describes a similar system. Both Gault and Galan teach close coupling between magnet and sensor and an externally mounted magnetic sender requiring penetration into the pressurized interior of the tire and rim by a pressure line.
U.S. Pat. No. 5,814,725 to Furuichi et al, discloses a mechanism that penetrates a tire rim wherein a piston-driven screw rotates a permanent magnet Magnetic field strength is measured by a stationary Hall-effect sensor that is mounted transversely to the magnet rotation axis. U.S. Pat. No. 6,182,514 to Hodges also discloses a magnet in a bellows that moves to change the magnetic field strength at an external magnetic intensity sensor or magnetic switch. U.S. Pat. No. 4,667,514 to Baer describes a similar arrangement. These types of device typically share the same problems as the other devices that depend on sensing magnetic field strength rather than direction.
U.S. Pat. No. 3,807,232 to Wetterhorn teaches a self-contained gauge comprising a permanent magnet attached in place of the conventional dial pointer of a Bourdon tube pressure gauge so that it rotates with pressure. A magnetic compass sensor is coaxially aligned to detect the rotational direction of the magnet and hence the pressure. U.S. Pat. No. 6,499,353 to Douglas et al. discloses a virtually identical Bourdon tube and coaxial magnetic compass apparatus to that of Wetterhorn wherein the sender and compass are separated by and are perpendicular to the wall of the pressure vessel. However, Bourdon tubes are complex, bulky, and are too fragile for road tire use. Bourdon tube forces are also weak. Bourdon tubes further lack the ability to support and rotate the larger magnets required for vehicular application. Furthermore, the requirement for coaxial alignment perpendicular to the tire wall is unacceptable to vehicle designers.
Several mechanisms besides Bourdon tubes have been disclosed for converting translational pressure or force urging into rotary motion via mechanical coupling. U.S. Pat. No. 5,103,670 to Wu describes the use of a screw to convert linear displacement from a conventional bellows to actuate a directly viewed rotary dial or pointer. U.S. Pat. No. 6,082,170 to Lia et al. describes a blood pressure apparatus that uses a diaphragm bellows and a compressible helical ribbon spring to rotate a dial pointer. None of these types of device employs magnetic coupling for remote sensing.
U.S. Pat. No. 2,722,837 to Dwyer teaches a pressure dial apparatus comprising a magnetic circuit with a permanent magnet translated by pressure coupled through a diaphragm along a high permeability helix. The helix and attached dial pointer rotate in accordance with the longitudinal position of the magnetic circuit along the helix. Various improvements and variations of this basic system are disclosed in a series of later patents assigned to Dwyer Instruments, Inc., etc. (e.g., U.S. Pat. No. 4,374,475 to Hestich, U.S. Pat. No. 4,890,497 to Cahill, U.S. Pat. No. 4,938,076 to Buchanan, etc.) None of these disclose rotating a magnet in response to pressure or employing a rotated magnetic field for remote pressure sensing.
Numerous devices include mechanisms moving a permanent magnet in response pressure or other force to induce a sensed effect in a material responsive to variation in magnetic field strength. For example, U.S. Pat. No. 4,006,402 to Mincuzzi, U.S. Pat. No. 4,843,886 to Koppers et al. and U.S. Pat. No. 4,627,292 to Dekrone, each teach a device based on either magnetoresistance and magnetic saturation. U.S. Pat. No. 4,339,955 to Iwasaki discloses a mechanism that exploits variation in the incremental permeability of a magnetically soft material. These devices sense field strength instead of direction. Devices based on the sensing the strength of a magnetic field rather than field direction typically require a narrow spacing between the sensing means and the translated magnet. They are very sensitive to changes in spacing, small misalignments, and extraneous magnetic fields. Accordingly, such devices generally require careful and extensive calibration before measurements are made, and are generally unacceptable for tire pressure reporting.
Still other concepts of remote pressure sensing involve a change the state indicator responding a preset pressure level. For example, U.S. Pat. No. 3,946,175 to Sitabkhan teaches switching a magnetically susceptible reed in response to pressure actuated displacement of a magnet. U.S. Pat. No. 5,542,293 to Tsuda et al. describes a conventional bellows actuated mechanism that uses a fixed and a moveable magnet to switch the orientation of a third magnet. U.S. Pat. No. 5,717,135 to Fiorefta et al. discloses use of magnetic coupling to switch the state of a transducer from producing to not producing a signal. These types of mechanisms do not produce a continuous output responsive to pressure.